Unpublished observations on the thecate stage of the dinoflagellate genus Pyrocystis by the late C. A. Kofoid and Josephine Michener

Abstract

Although the motile stage of marine dinoflagellates referable to Dissodinium Pascher is well known, comparable stages are much less well known in Pyrocystis Wy.–T. ex J. Murray. The author illustrates the thecal features of the latter with the aid of previously unpublished figures by two prominent early observers and material from the Indian Ocean. Thecate motile cells arise in pairs within planktonic secondary cysts (smaller than the primary cysts). Their tabulation is similar to the genus Gonyaulax Diesing although the hypothecae may exhibit fewer component plates. Thecate cells have been observed in the type species, P. pseudonoctiluca, P. fusiformis forma fusiformis and P. fusiformis f. biconica. The problem of generic distinction based on life cycles is discussed. The genus Pyrocystis is emended to include the nature of the thecate motile stage. Pyrocystis is maintained as separate from Gonyaulax in view of the presence of two distinctive, consecutive planktonic cyst stages, in the life cycle of the former. The genus Dissodinium has similar cyst stages by which it is distinguished from the genera Gymnodinium and Gyrodinium, the lack of strongly tabulated Gonyaulax-like thecae in the motile stages serving to separate it from Pyrocystis.